Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. Implantable medical devices (IMDs) include implantable cardiac devices such as, for example, pacemakers, cardioverters and defibrillators. The term “implantable cardioverter defibrillator” or simply “ICD” is used herein to refer to any implantable cardiac device or implantable cardioverter defibrillator (“ICD”).
ICDs are typically implanted in patients suffering from potentially lethal cardiac arrhythmias. Arrhythmia, meaning “without rhythm,” denotes any variance from normal cardiac rhythm. Heartbeat irregularities are fairly common and many are harmless. A severe heartbeat irregularity known as ventricular tachycardia refers to a runaway heartbeat.
Fibrillation is an irregular rhythm of the heart caused by continuous, rapid, electrical impulses being emitted/discharged at multiple locations known as foci in the heart's atria and ventricles. Because a fibrillating heart is unable to properly pump blood through a patient's body, the longer a patient is in fibrillation, the greater the potential damage that can occur to the patient's heart. Thus, after the start of fibrillation, it is preferable to apply defibrillating therapy to the patient as soon as possible. An ICD is designed to apply such therapy automatically and quickly to minimize damage to the heart.
An ICD monitors cardiac activity and decides whether electrical therapy is required. For example, if a tachycardia is detected, pacing or cardioversion therapy may be used to terminate the arrhythmia. If fibrillation is detected, defibrillation is the only effective therapy. Both cardioversion and defibrillation require that a high voltage shock be delivered to the heart.
Typical ICDs include a set of electrical leads, which extend from a sealed housing into the walls of a heart after implantation. Within the housing are a battery for supplying power, a capacitor for delivering bursts of electric current through the leads to the heart, and monitoring circuitry for monitoring the heart and determining when, where, and what electrical therapy to apply. The monitoring circuitry generally includes a microprocessor and a memory that stores instructions not only dictating how the microprocessor controls delivery of therapy, but also controlling certain device maintenance functions, such as maintenance of the capacitors in the device. One example of an ICD is shown in U.S. Pat. No. 7,835,788, issued Nov. 16, 2010, the disclosure of which is incorporated herein in its entirety as though set forth in full below.
An implantable pulse generator feedthru is used for an electrical pathway extending between the electrically conductive lead securing components of a header of the pulse generator and the electrical components, such as an output flex, hybrid, etc., hermetically sealed in the housing or can of the pulse generator.
Feedthrus are mounted in the wall of the housing or can and include feedthru wires extending through the feedthrus. Feedthrus provide insulated passageways for feedthru wires, such as platinum iridium (Pt/Ir) wires, through the wall of the can. The header ends of the feedthru wires are electrically connected to connector blocks that mechanically and electrically couple with connector ends of implantable medical leads, and the can ends of the feedthru wires are electrically connected to the electrical components housed in the can of the pulse generator.
Feedthrus may include a filter element to filter out unwanted signals, such as electromagnetic interference (“EMI”). EMI feedthru filters used in ICDs require a high radio frequency (RF) performance, specifically a high voltage rating in a small feedthru housing assembly. The RF performances include high series resonant frequency (SRF), wide band width and high Q, etc.
In order to meet these specifications, not only the feedthru housing assembly needs to be optimized to accommodate the large valued capacitors, but also the large valued capacitors themselves need to be optimized.
In practice, RF capacitors used in an RF transceiver or RF decoupling circuit only have a value from about 0.5 pico Farads (pF) to several hundred pF. Capacitors used in feedthru filters will usually have a value range from about 1 to 5 nano-Farads, which is 10 times larger than regular RF capacitors.
Fundamentally these large valued capacitors (e.g., from 1-5 nF) are composed of multi-layer capacitors (MLC), wherein several single layer capacitors inside the capacitor body are vertically stacked and connected in parallel to make a larger valued capacitor. The larger the capacitance values that are required, the more single layer capacitors that need to be stacked.
In practice, a single layer capacitor contains some series parasitic inductance which will downgrade the RF performance. In particular, it will generate an SRF frequency point. At operating frequencies above the SRF, the capacitor will behave like an inductor and downgrade the RF performance of the capacitor.
When implementing a large valued MLC capacitor, the parasitic inductances accumulate when multiple single layer capacitors are stacked inside a multi-layer capacitor body.
A large valued capacitor will have more parasitic series inductance and generate a very low SRF frequency point. This will downgrade the RF performance of the capacitor. The larger the value of a capacitor, the higher the parasitic series inductance will be. In the worst case scenario, the SRF will be shifted to a very low frequency; this will cause the capacitor to work only at a low frequency band.
Several techniques, designs and processes have been developed to reduce the parasitic series inductance in a large valued RF capacitor. One such example is described in U.S. Pat. No. 7,623,336. These large valued RF capacitors are specially designed and implemented such that the SRF frequency can extend to very high frequency range. However, the costs of these custom made capacitors are significantly higher than standard capacitors.